I live in a college apartment complex, so I chain mine to a pole with a thick piece of cable and a padlock. I figure that, as has been said before, if they really want it then they'll get it, but I might as well make them work for it.
My old security system was that mine was chained up and my neighbor's GSXR-750 wasn't, but then he moved. Regardless, I think the main point is just making it less appealing to take- locking the handlebars, using a caliper lock, etc.
When my budget grows and I move to a bigger city, however, a two-way alarm will certainly be installed!